This is exactly what I did:
Originally Posted by eblade
1) I went to this site: Welcome | Straight Talk and bought an AT&T compatible SIM.
I tried going into 2 of my local Wal-Marts, but apparently, they don't carry the BYOD device SIM kit in-store; you'll have to buy the kit online. For now, they offer free 3-day ground shipping on FedEx (only took me 2 days to get mine).
2) The kit will come with an activation card, service guide and, obviously, SIM card. The red activation card will guide you through the entire process of getting the SIM activated. The process, including porting my number, took no more than 15 minutes.
3) When I was done, I put the SIM in my Pre2 and viola! I was on Straight Talk.
I can't recommend going the "take-the-SIM-out-of-an-Android-phone" route as it would probably be more difficult. The BYOD promotion path was a lot easier. However, before the promotion, this is what people were doing before (and it worked). However, YMMV.
As for picking up a prepaid Android phone as a dev'ing one, I used to develop for an assortment of prepaid Android phones (Huawei Ascend, Optimus V and Motorola Triumph). It was fun, but was difficult due to manufacturers not releasing source code. Sometimes, they wouldn't even release their changes to GPL code (i.e. kernel source). If you're looking to develop apps, then it doesn't sound like a bad idea. There's a nice community of Android devs on Github. You can see my Android adventures here: http://github.com/ikarosdev